1. Field of the Invention
This invention relates to a ferroelectric liquid crystal material. More particularly it relates to a ferroelectric liquid crystal composition comprising achiral smectic liquid crystal compound(s) and optically-active compounds and having quick response properties, and to a light-switching element containing the composition.
2. Description of the Related Art
Liquid crystal compounds have been broadly used as materials for display elements, but most of such liquid crystal display elements are of the TN display mode and as liquid crystal materials, those belonging to nematic phase have been used.
The TN display mode has characteristics resulting in the eyes are not being tired due to non-emissive mode and the power consumption is very small, but to the contrary has drawbacks that the response is slow and the viewing angle of the display is narrow.
However, recently the mode has been used in flat displays, and thus quick response properties and a broad viewing angle have been particularly required therefor.
In order to satisfy such requirements, improvement in liquid crystal materials has been attempted. However, as the compared with other emissive mode displays such as electroluminescent display, plasma display, etc., the TN display mode is observed to have large differences from the former in the aspects of response time and width of viewing angle.
In order to make use of the specific features of non-emissive mode and a small power comsumption and yet to secure response properties matching the emissive mode display, development of a novel liquid crystal display mode in place of the TN display mode is indispensable.
As one of such attempts, a display mode utilizing the light-switching phenomenon of ferroelectric liquid crystals was proposed by N.A. Clark and S.T. Lagewall (see Applied Physics Letters, Vol. 36, p. 899 (1980)).
As to ferroelectric liquid crystals, their presence was announced by R.B. Meyer et al for the first time (see Journal de Physique, Vol. 36, p. 69, 1975), and in the aspect of liquid crystalline structure, their phases belong to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase, chiral smectic H phase, chiral smectic J phase or chiral smectic K phase (hereinafter abbreviated to Sc* phase, S.sub.I * phase, S.sub.F * phase, S.sub.G * phase, S.sub.H * phase, S.sub.J * phase or S.sub.K * phase, respectively).
When the light-switching phenomenon of ferroelectric liquid crystals is applied to display elements, the resulting display elements have two superior specific features as compared with the TN display mode. The first specific feature consists in that the elements respond at a very high speed and the response is more rapid by about 10.sup.3 to 10.sup.4 times than elements of TN display mode. The second specific feature consists in a memory effect which makes multiplex drive easy coupled with the above quick response properties.
Among the chiral smectic phases, the Sc* phase has now been particularly noted.
When ferroelectric liquid crystals are applied to display devices, the following conditions required for liquid crystal materials are exemplified:
1 A Sc* phase is exhibited within a broad temperature range including room temperature (at least 0.degree. to 50.degree. C.).
2 Display elements of 640.times.400 lines or more are required to exhibit a response time of 100 .mu.sec or less.
The response time (.tau.) of a ferroelectric liquid crystal in an electric field (E) is expressed by the equation ##EQU1## wherein .eta. is viscosity and Ps is spontaneous polarization.
Thus in order to realize the quick response properties, liquid crystal materials are required to exhibit a large spontaneous polarization value.
3 In order to realize the memory effect according to N.A. Clark et al, it is necessary to bring the cell gap (d) into the helical pitch (P) or less and untwist the helix (see Appl. Phys. Lett. 36 (1980) 899). Thus, in order to utilize a cell of having a thick cell gap which is easy to prepare, it is necessary to prolong the helical pitch of ferroelectric liquid crystals. 4 The aligning state of ferroelectric liquid crystals varies depending upon the phase series of liquid crystal materials, and at present, in the case of the aligning technique employed for TN liquid crystal materials (surface treatment method), liquid crystal materials having a smectic A phase and cholesteric phase (hereinafter abbreviated to S.sub.A phase and N* phase, respectively) on the higher temperature side relative to Sc* phase have been regarded to afford the best aligning state. Namely, the phase series of ferroelectric liquid crystals is preferred to be isotropic liquid (hereinafter abbreviated to Iso).fwdarw.N*.fwdarw.S.sub.A .fwdarw.Sc* (for example, see Japanese patent application laid-open No. Sho 61-250086).
Further, among liquid crystal materials having such a phase series, those having a N* phase of a longer pitch have been regarded to exhibit a better aligning state (for example, see Japanese patent application laid-open No. Sho 61-255323).
Besides the above-mentioned conditions, there are various requirements for the tilt angle (.theta.) of liquid crystal molecules, etc.
As to conventional ferroelectric liquid crystal materials, even when only the temperature range is referred to, practical materials are few, and it is the present status that materials satisfying all of the above conditions and durable to practical use are almost none.
For example, Japanese patent application laid-open No. Sho 61-291,679 and PCT international application laid-open No. WO 86/06401 (pamphlet) disclose ferroelectric liquid crystal compositions obtained by combining an achiral phenylpyrimidine compound having a smectic C phase (hereinafter abbreviated to Sc phase) with an optically active compound, which compositions exhibit a Sc* phase within a broad temperature range including room temperature and a phase series of I.sub.so .fwdarw.N*.fwdarw.S.sub.A .fwdarw.Sc*. However, the ferroelectric liquid crystal compositions disclosed in such publications satisfy the above-mentioned requirements in the aspect of the temperature range of the Sc* phase and the phase series, but the response time is 300 to 500 .mu.sec (e.g., see ferroelectric liquid crystal compositions described in Examples 1 and 2 of the above Japanese patent application laid-open No. Sho 61-291679 or Examples 45 and 46 of the above PCT international application laid-open No. WO 86/06401) and hence the response time is not practical.
As apparent from the foregoing, it is the present status that ferroelectric liquid crystal materials satisfying all of the above-mentioned conditions and hence applicable to display devices are almost none.